Integrated circuitry fabrication typically involves lithographic processing to transfer patterns from an imaging layer to an underlying substrate material which will form part of the finished circuitry. One example process is photolithography, for example wherein the imaging layer comprises photoresist. A form of radiant energy is passed through a radiation-patterning tool and onto the photoresist. The radiation-patterning tool is commonly referred to as a mask, photomask, or reticle. The term “photomask” has traditionally been used to refer to masks which define a pattern for an entire side of a substrate, while the term “reticle” traditionally refers to a masking tool which defines a pattern for only a portion of a side of a substrate. A reticle typically encompasses multiple die/chip areas of the substrate being patterned. The reticle and substrate bearing the photoresist are moved relative to one another such that multiple discrete exposures of the substrate through the reticle completely pattern the entire side of the substrate. The term “mask” as used herein, is generic to any radiation-patterning tool, whether existing or yet-to-be developed, and whether defining a pattern on a portion of or on an entire side of a substrate.
It is desirable that the pattern in the mask be replicated onto the substrate being patterned as closely as possible and in the desired location. Three variables or parameters which impact the pattern which is formed on the substrate are dose, focus, and overlay/x-y alignment. Dose may be considered as the quanta of energy passed through the mask onto the substrate being patterned, and is largely determinative of feature dimensions in the lateral or horizontal direction. Focus is the measure of the distance of the substrate being exposed relative to the lens through which the radiation is passed, and is largely determinative of feature aspects in profile or vertical direction. Overlay is a measure of accuracy of alignment of the substrate and mask relative to x and y axes.
Photolithography equipment in production is periodically calibrated to achieve acceptable dose, focus, and overlay to replicate the mask patterns onto production substrates. Registration and other monitoring marks are typically provided within the mask outside of the areas where circuitry is being fabricated. These monitoring marks may be used in calibration of the production photolithography equipment and to determine accepted or rejected patterning with respect to the production substrates being processed.